Preparation of saturated perfluoropolyethers has traditionally been limited because of the lack of versatile synthetic techniques. A successful synthesis is the polymerization of perfluoroepoxides, particularly hexafluoropropylene oxide and tetrafluoroethylene oxide. W. T. Miller, U.S. Pat. No. 3,242,218. This synthetic procedure involves a three-step scheme for production of the polymer involving oxidation of perfluoroolefins to perfluoroepoxides, followed by anionic polymerization to acyl fluoride terminated perfluoropolyethers and then replacement of the acyl fluoride end groups with perfluoroalkyl groups by decarboxylation reactions or by chain coupling photolytic decarboxylation reactions.
The procedure, however, allows little control over the molecular weight distribution of the product. Typically, a distillation cut is taken if a specific molecular weight range is needed. When tetrafluoroethylene oxide is polymerized, little if any low molecular weight fluids are obtained; the majority of the product is a higher molecular weight solid. Conversely, the polymerization of perfluoropropylene oxide gives only a liquid; no products are isolated with a sufficiently high molecular weight to be solid.
An alternate synthetic method for the production of perfluoropolyethers involves the ultraviolet photolysis of tetrafluoroethylene and/or hexafluoropropylene in an inert solvent in the presence of oxygen, D. Sianesi and R. Fontanelli, British Patent 1,226,566. The multistep process yields an acyl fluoride terminated polymer which contains unstable peroxidic linkages in addition to difluoromethylene oxide and tetrafluorethylene oxide (or hexafluoropropylene oxide) repeating units. Treatment of the polymer at elevated temperatures and with fluorine gas gives a stable polymer containing only perfluoroalkyl end groups. Once again, it is very difficult to control the molecular weight of the polymer product. The product can be separated into various fractions based on vapor pressure.
Lagow and Gerhardt (U.S. Pat. No. 4,523,039) describe a method for preparing perfluoropolyether oligomers. The method entails prefluorination of a high molecular weight polyether followed by a fluorination of the resulting partially fluorinated polyether at elevated temperatures which results in some bond breakage as the remaining hydrogens are being replaced. An elevated temperature is chosen so that sufficient amount of bond breakage occurs. For most materials a temperature between 55.degree. and 210.degree. C. is preferred (see column 4, line 32). However, perfluoropolyethers (having essentially no residual hydrogen atoms) are generally stable in fluorine at temperatures within this range. See e.g., British Patent No. 1,226,566 (the Fomblin Y.TM. perfluoropolyethers stable in fluorine up to 320.degree. C.); U.S. Pat. No. 3,242,218 (polytetrafluoroethylene oxide stable in fluorine at 185.degree.-190.degree. C.). Thus, the strategy of Lagow and Gerhardt involves a "prefluorination" period with conditions selected to maintain the structural integrity of the polymer followed by a "fragmentation" period where the fluorination takes place at a sufficiently high temperature to cause some fragmentation as the fluorinated polymer becomes perfluorinated.
In order to obtain a narrow molecular weight distribution, the hydrogen left on the polymer following the pre-fluorination step must be randomly distributed. This is difficult to accomplish in certain instances. Material of particle sizes as small as 200 mesh are fluorinated by first adding fluorine to the surface followed by diffusion of fluorine through the fluorinated layer into the interior of the particle. The rate of reaction is limited by the rate at which the fluorine can diffuse into the interior of the particle. Upon completing the pre-fluorination phase of the reaction, each particle contains an essentially perfluorinated shell with an interior which is rich in hydrogen. The fragmentation-fluorination step has no affect on the shell which is stable in fluorine up to its thermal decomposition temperature (350.degree. C.) while the hydrogen-rich core essentially burns. The net result is an abundance of inert solids as well as CF.sub.4 and other very small fragments. In order to optimize the yield of an intermediate molecular weight fraction with commercial value one would need to eluorinate very small particles--a starting material which is generally not available.
There exists a need for a convenient means to alter the molecular weight of a perfluoropolyether polymer regardless of the method by which it is made.